1. Field of the Invention
The present invention relates to a technique of driving a display panel composed of display elements having a memory function, and particularly, to a method of and an apparatus for driving an alternating current (AC) plasma display panel (PDP), which provides multiple intensity levels and adjusts the luminance of a full color image plane.
2. Description of the Related Art
In the AC PDP, voltage waveforms are alternately applied to sustain two discharge electrodes, to maintain discharge and to display an image by emission. Each shot of discharge lasts several microseconds after the application of a pulse. Ions, i.e., positive charges produced by the discharge are accumulated over an insulation layer on an electrode of negative voltage. Electrons, i.e., negative charges produced by the discharge accumulate over an insulation layer on an electrode of positive voltage.
At first, a pulse (a write pulse) having a high voltage (a write voltage) is applied to cause discharge and produce wall charges. Thereafter, a pulse (a sustain discharge pulse) having a low voltage (a sustain discharge voltage) whose polarity is opposite to that of the high voltage and which is lower than the high voltage is applied to enhance the accumulated wall charges. As a result, the potential of the wall charges with respect to a discharge space exceeds a discharge threshold voltage to start discharging. In this way, once the wall charges are accumulated in a cell by the write discharge, the cell can continuously discharge if sustain discharge pulses, having opposite polarities, are alternately applied to the cell. This phenomenon is called a memory effect or a memory drive. The AC PDP enables various image data to be displayed by utilizing such a memory effect.
These kinds of AC PDPs are classified into a two-electrode type employing two electrodes for carrying out selective discharge (addressing discharge) and sustain discharge, and a three-electrode type additionally employing a third electrode to carry out addressing discharge. Among such AC PDPs, in the AC PDP displaying color images (full color images) with multiple intensity levels, i.e., a color PDP, a phosphor located within each cell is excited by ultraviolet rays generated due to a discharge between different kinds of electrodes. However, this phosphor is relatively fragile against a hitting of ions, i.e., positive charges are also generated due to the discharge. The former two-electrode type PDP has a construction such that the ions collide directly with the phosphor, and therefore the life of the phosphor is likely to become shortened. On the other hand, in the latter three-electrode PDP, a surface-discharge with high voltage is carried out between a first-electrode and a second electrode, each located in the same plane. In such a construction, the phosphor at the side of the third electrode is avoided from the direct and strong bombardment of ions, and consequently a life of the phosphors is likely to become longer. Namely, the three-electrode PDP is advantageous in displaying color (full color) image with multiple intensity levels. Accordingly, as the color PDP, the three-electrode type is currently used. The amount of emission (luminance) of the three-electrode PDP is determined by the number of pulses applied to the PDP.
FIG. 1 is a plan view schematically showing a conventional three-electrode and surface-discharge PDP.
In FIG. 1, numeral 1 is a panel, 2 is an X electrode, 3.sub.1, 3.sub.2, - - - , 3.sub.K, - - - , 3.sub.1000 are Y electrodes, and 4.sub.1, 4.sub.2, - - - 4.sub.K, - - - 4.sub.M are addressing electrodes. A cell 5 is formed at each intersection where a pair of the X and Y electrodes crosses one of the addressing electrodes, to provide M.times.1000 cells 5 in total. Numeral 6 is a wall for partitioning the cells 5, and 7.sub.1 to 7.sub.1000 are display lines.
FIG. 2 is a sectional view schematically showing the basic structure of the cell 5. Numeral 8 is a front glass substrate, 9 is a rear glass substrate, 10 is a dielectric layer for covering the X electrode 2 and Y electrode 3.sub.k, 11 is a protective film of an MgO film or the like, 12 is a phosphor, and 13 is a discharge space.
FIG. 3 shows the conventional PDP of FIG. 1 and its peripheral circuits. Numeral 14 is an X driver circuit for supplying a write pulse and a sustain discharge pulse to the X electrode 2, 15.sub.1 to 15.sub.4 are Y driver ICs for supplying addressing pulses to the Y electrodes 3.sub.1 to 3.sub.1000, 16 is a Y driver circuit for supplying pulses other than the addressing pulses to the Y electrodes 3.sub.1 to 3.sub.1000, 17.sub.1 to 17.sub.5 are addressing driver ICs for supplying addressing pulses to the addressing electrodes 4.sub.1 to 4.sub.M, and 18 is a control circuit for controlling the X driver circuit 14, Y driver ICs 15.sub.1 to 15.sub.4, Y driver circuit 16, and addressing driver ICs 17.sub.1 to 17.sub.5.
FIG. 4 is a waveform diagram showing a first conventional method of driving the PDP of FIG. 1. More precisely, this figure shows a drive cycle of a conventional "sequential line driving and self-erase addressing" method.
This method selects one of the display lines to write display data thereto during the drive cycle. The Y electrode of the selected line is set to a ground level (GND: 0 V), and the Y electrodes of the other display lines (unselected lines) are set to a potential level of Vs. A write pulse 19 having a voltage of Vw is applied to the X electrode 2, to discharge all cells of the selected line. At this time, a voltage difference between the X and Y electrodes of the selected line is Vw, and a voltage difference between the X and Y electrodes of the unselected lines is Vw-Vs. By setting Vw&gt;Vf&gt;Vw-Vs (where Vf is a discharge start voltage), all cells of the selected line will discharge.
As the discharge progresses, the protective film, 11, e.g., an MgO film over the X electrode 2 of the selected line accumulates negative wall charges, and the MgO film over the Y electrode of the selected line accumulates positive wall charges. Since the polarities of these wall charges are to reduce an electric field in the discharge space, the discharge quickly converges and ends within about a microsecond.
Sustain discharge pulses 20 and 21 are alternately applied to the X and Y electrodes of the selected line, so that the accumulated wall charges are added to the voltages applied to the electrodes, to repeat sustain discharge in cells except those that are not turned ON (not in light emission).
For the cells that are not turned ON, the first sustain discharge pulse 20a is applied to the X electrode 2, to accumulate positive wall charges in the MgO film over the X electrode 2 of the selected line, and negative wall charges in the MgO film over the Y electrode of the selected line. In synchronism with the first sustain discharge pulse 21a, applied to the Y electrode of the selected line, an addressing pulse (an erase pulse) 22 having a positive voltage of Va is selectively applied to the addressing electrodes of the cells not to be turned ON.
At this time, sustain discharge occurs in every cell of the selected line, and in particular, the cells that have received the positive addressing pulse 22 through the addressing electrodes cause discharge between the addressing electrodes and the Y electrode, to excessively accumulate positive wall charges in the MgO film over the Y electrode.
If the voltage Va is set such that the voltage of the wall charges exceeds the discharge start voltage, the voltage of the wall charges induces discharge when the external voltages are removed, i.e., when the potential of the X and Y electrodes is returned to Vs and that of the addressing electrodes to GND. This causes self-erase discharge to dissipate the wall charges in the cells not to be turned ON. Accordingly, from this moment, the sustain discharge pulses 20 and 21 will never cause sustain discharge in the cells not to be turned ON.
For the cells to be turned ON, the erase pulse (addressing pulse) 22 is not applied to the corresponding addressing electrodes, to cause no self-erase discharge in these cells. Accordingly, the sustain discharge pulses 20 and 21 repeatedly cause sustain discharge in the cells turned ON. Numeral 23 is a sustain discharge pulse applied to the Y electrodes of the unselected lines.
In this way, display data are written to a selected line in each drive cycle. In the example mentioned above, the write operation is carried out on the display lines line by line. FIG. 5 is a time chart showing the write operation. In the figure, "W" is a write cycle, "S" is a sustain discharge cycle, and "s" is a sustain discharge cycle of a preceding frame (field).
FIG. 6 is a waveform diagram showing a second conventional method of driving the PDP of FIG. 1. More precisely, the figure shows a frame of a conventional "separately addressing and sustain-discharging type self-erase addressing" method.
This method divides the frame into a total write period, an addressing period, and a sustain discharge period. During the total write period, the potential of the Y electrodes 3.sub.1 to 3.sub.1000 is set to GND, and a write pulse 24 having a voltage of Vw is applied to the X electrode 2, to cause discharge in all cells of all of the display lines. The Y electrodes 3.sub.1 to 3.sub.1000 are then returned to Vs, and a sustain discharge pulse 25 is applied to the X electrode 2, to cause sustain discharge in every cell.
During the addressing period, display data are sequentially written to the display lines from the display line 7.sub.1. At first, an addressing pulse 26.sub.1 having a level of GND is applied to the Y electrode 3.sub.1, and an addressing pulse 27 having a voltage of Va is applied to selected ones of the addressing electrodes 4.sub.1 to 4.sub.M that correspond to cells not to be turned ON of the display line 7.sub.1, to cause self-erase discharge in these cells. This completes the write operation of the display line 7.sub.1.
The same operation is carried out for the display lines 7.sub.2 to 7.sub.1000 sequentially, to write new data to all of the display lines 7.sub.1 to 7.sub.1000. Numerals 26.sub.2 to 26.sub.1000 are addressing pulses sequentially and separately applied to the Y electrodes 3.sub.2 to 3.sub.1000.
During the sustain discharge period, sustain discharge pulses 28 and 29 are alternately applied to the Y electrodes 3.sub.1 to 3.sub.1000 and X electrode 2, to carry out sustain discharge to display an image for the frame. According to the separately addressing and sustain-discharging type self-erase addressing method, the length of the sustain discharge period determines luminance.
The separately addressing and sustain-discharging type self-erase addressing method, therefore, is used for displaying an image with multiple intensity levels. For example, this method is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 4-195188. FIG. 7 shows a method of realizing 16 intensity levels as an example of the multiple intensity level displaying technique. In this example, a frame is divided into four subframes (subfields) SF1, SF2, SF3, and SF4.
In the subframes SF1, SF2, SF3, and SF4, total write periods Tw1, Tw2, Tw3, and Tw4 are equal to one another, and addressing periods Ta1, Ta2, Ta3, and Ta4 are also equal to one another. Sustain discharge periods Td1, Td2, Td3, and Td4 are at a ratio of 1:2:4:8. The 16 intensity levels are achieved by selectively combining the subframes to turn cells ON.
FIG. 8 is a waveform diagram showing a third conventional method of driving the PDP of FIG. 1. More precisely, the figure shows a drive cycle of a conventional "sequential line driving and selective-write addressing" method. In this method, generally, a negative voltage (-V.sub.s) is applied to X and Y electrodes. Therefore, in FIG. 8, each potential of X and Y electrode is set to GND level or (-V.sub.s).
This method applies a narrow erase pulse 30 to the Y electrode of a selected line, to turn OFF cells that are ON. An addressing pulse (a write pulse) 31 of a voltage (-V.sub.s) is applied to the Y electrode of the selected line, while the potential of the Y electrodes of the other unselected lines is kept at a ground (GND) level. An addressing pulse (a write pulse) 32 having a voltage of Va is applied to the addressing electrodes of cells to be turned ON, to discharge these cells.
Sustain discharge pulses 33 and 34 are alternately applied to the X electrode and the Y electrode of the selected line, to repeatedly cause sustain discharge to write display data to the selected display line. Numeral 35 is a sustain discharge pulse applied to the Y electrodes of the unselected lines.
However, the following problems have existed in the above-mentioned driving methods of PDP (prior arts).